1. VENTILATION NOISE

2. Components Air from outside Conditioned air to rooms via ducts
Distribution system
Air-handling Unit
Fan with filters, heaters, coolers, to condition the air.
Rectangular or circular

3. Noise Sources and Transmission Paths
TRANSMISSION VIA STRUCTURE
Case Radiation
&
Vibration
Vibration
Via Supports
Duct Breakout
(SECONDARY PATHS)
FAN
(PRIMARY
SOURCE)
TRANSMISSION THROUGH DUCTWORK
(PRIMARY PATH)
SOUND ENERGY REACHING THE OCCUPANTS OF THE BUILDING
TRANSMISSION TO
OUTSIDE
REGENERATED NOISE
DUE TO TURBULENCE
(SECONDARY SOURCES)

4. Dealing with Secondary Sources & Paths
These can be minimised by:-
Minimising turbulence and reducing velocities
Enclosing the fan
Cladding ductwork
Flexible couplers between fan and ductwork
Anti-vibration mounts for the fan
Anti-vibration hangers for the ductwork
This leaves primary source and path as our main concern – and can be split into the atmospheric side and the room side (on which we will concentrate)

5. Fan Noise Calculation (Room Side)
Methodology
start with the fan sound power level, Lw and correct this for attenuations along the ductwork and into the room.
Attenuation of straight duct
Reflections at bends
Attenuation at branches
Reflections from the end of the duct
Lw at the diffuser = Fan Lw – attenuations
Lp at a position in the room depends on Lw at diffuser and the effect of distance (diffuser to position) and room absorption.
Note:- these calculations are for each octave band

6. Regenerated Noise
Note that in high velocity systems there will be secondary noise sources at bends and dampers within the ductwork.
These will need to be considered separately and the appropriate attenuations applied from where they occur within the ductwork to the room as before.
The levels in the room from the secondary sources must be added to that from the fan to obtain the overall level in the room.
Secondary sources are ignored in this simple method.

7. Fan Power
The best data is that supplied by the manufacturer based on tests of octave band sound power levels in a special reverberation chamber.
Alternatively, a generic formula based on the power of the fan (kW), the flow rate (Q), and the pressure produced (P), although it is is less accurate.
Lw = kW + 10logQ + 10logP + C
C is a constant depending on the fan efficiency
Note that this is the total sound power level and must be split between room and atmospheric sides (-3 dB?)
And, corrections applied to get the octave band levels

8. Fan Performance Curve Pressure Developed, P (Pa)
Fan Curve (flow rate achieved when the
fan is developing a given pressure)
Operating Point
(pressure and flow when this fan is used in this ductwork system
Pressure Developed, P (Pa)
System Curve
(pressure required for a given
flow rate in the ductwork)
Volume Flow Rate, Q (litres/s)

9. The test to determine the sound power level is similar to the one you performed in the reverberant chamber at UWE

10. Typical Fan Data Sheet
Overall sound power level (A weighted) ,LwA at various fan operating points
Fan
Curve
Pressure developed,P (Pa)
Volume flow rate, Q (m3/s)

11. Corrections for the Octave Band Levels

12. Attenuation of Sound Power Level
straight duct
(energy removed to support duct wall vibration)
bends
(certain frequencies are reflected back at bends)
plenum chambers / changes in x-section
(sound is reflected if the cross section changes abruptly)
branches
(sound power is shared if the duct splits into branches)
End reflection
(due to impedance mismatch)

13. Attenuation -- Straight Duct
Mainly due to duct wall flexing which takes energy out
of the passing sound wave
Note:- less attenuation per metre for large ducts
less attenuation per metre for circular ducts
less attenuation per metre for high frequencies

14. Attenuation of bends
Long wavelengths negotiate bends with little reflection
Short wavelengths travels by cross reflections and go round bends easily
When the duct width is ½ a wavelength sound is strongly reflected back at bends

15. Plenum Chamber
Used to distribute the air and can be used in noise control
Attenuation depends on location of inlet and outlet, size
and absorption of internal surfaces.

16. Attenuation --- Branches
Sound power is split between the branches
Use graph or equation
Atten. = 10lg(A1/(A1+A2))
A1 is the cross sectional area of the branch in question,
A1 + A2 is the total cross sectional area of all the branches

17. End Reflection Low frequency sound is reflected back due to the change
from an enclosed duct to the larger room (impedance mismatch)

18. Lp in the Room
Sound Power Level, Lw at the diffuser is the fan Lw minus the attenuations plus any regen noise.
Q = geometric directivity , 4, or 8
Rc = room constant (m2)
r = distance (diffuser to position)
usually 1m for the worst position (closest to the diffuser)

19. Is it a problem?
Plot the octave band room Lp’s on NR curves to find the attenuation required.
The difference between the Lp’s and the target NR gives the additional attenuation required
Say target is NR35
NR35

20. Acoustic Attenuators
Simplest is to line the duct with sound absorbing material
More effective if we split the airway to make a number of narrower ducts
Typical performance

21. Typical attenuators (silencers)
Louvre Circular Pod Rectangular Splitter